gcc -Wclobbered in PostgresMain

Hi, hackers!

While analyzing -Wclobbered warnings from gcc we found a true one in
PostgresMain():

postgres.c: In function ‘PostgresMain’:
postgres.c:4118:25: warning: variable
‘idle_in_transaction_timeout_enabled’ might be clobbered by ‘longjmp’ or
‘vfork’ [-Wclobbered]
4118 | bool idle_in_transaction_timeout_enabled =
false;
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
postgres.c:4119:25: warning: variable ‘idle_session_timeout_enabled’
might be clobbered by ‘longjmp’ or ‘vfork’ [-Wclobbered]
4119 | bool idle_session_timeout_enabled = false;
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

These variables must be declared volatile, because they are read after
longjmp(). send_ready_for_query declared there is volatile.

Without volatile, these variables are kept in registers and restored by
longjump(). I think, this is harmless because the error handling code
calls disable_all_timeouts() anyway. But strictly speaking the code is
invoking undefined behavior by reading those variables after longjmp(),
so it's worth fixing. And for consistency with send_ready_for_query too.
I believe, making them volatile doesn't affect performance in any way.

I also moved firstchar's declaration inside the loop where it's used, to
make it clear that this variable needn't be volatile and is not
preserved after longjmp().

Best regards,

--
Sergey Shinderuk https://postgrespro.com/